Contact structures physically separating the current carrying and arcing functions of the contacts are known. The most common usage of this type of contact arrangement is in DC contactor design, employing single make type contact action. When the switch is turned ON, the arcing contact closes first followed by closure of the main contact in parallel with the arcing contact. When the switch is turned OFF, the main contact opens first followed by opening of the arcing contact, such that the latter experiences an arcing. This dual contact action is typical in DC contactors because of the high current loads required.
Double-break-and-make bridging contact structures are typical in AC contactor design. The double-break bridge structure is used to meet the particular requirements of AC interruption with a very simple, fairly compact, low cost device structure. In bridging type contact structures, it is common to employ silver contacts which are sized large enough to withstand any arcing, and have enough stock to afford a desired life rating in spite of deterioration caused by arcing.
In view of anticipated continuing increases in silver costs, it is a purpose of the present invention to reduce silver usage in bridging type contact structures, including AC contactors. A substantial reduction, for example 90-95%, in silver usage has been achieved by a particular contact structure affording dual contact action with double-break main contacts and couble-break arcing contacts.
The main contacts are rigidly fixed and comprise a thin stock of silver. The arcing contacts are likewise rigidly fixed and comprise copper cadmium oxide or thicker stock.
A bridging contact structure with double-break main contacts and double-break arcing contacts is shown in Terracol et al U.S. Pat. No. 4,039,983. The auxiliary arcing contacts are mounted to expandable loops electrodynmically operated.